This is an interesting way to monitor your speakers. [Keith] put together this speaker line monitor after a commenter requested it on his blog. It’s designed to check for power on the speaker line and drive a logic/led output. Apparently there’s some risk of shorting your amp, so he’s planning to redesign the input stage. Still, it’s neat little hack to keep an eye on things. Personally, I’d just put em on their own class A amp and automate the power switching, but there are plenty of situations where this could be useful.